1. Field of the Invention
The present invention relates generally to improvements in a radio communications system, and more specifically to an apparatus and operation method of implementing improved diversity reception using maximum-ratio combining techniques in a high-frequency digital radio communications system. The present invention is highly suited for being applied to digital line-of-sight microwave terminals merely by way of example.
2. Description of the Related Art
As is well known in the art, diversity reception is based on the fact that the radio signals arriving at a point of reception over separate paths may have non-correlated signal levels. More simply, at one instant of time a signal on one path may be in a condition of fade while the identical signal on another path may not.
Before turning to the present invention, it is deemed preferable to briefly describe, with reference to FIG. 1, a conventional diversity reception technique using a well-known technique of maximum-ratio combining.
FIG. 1 is a block diagram showing a conventional front section 8 of a diversity receiver, which section precedes a demodulator (not shown) provided for demodulating the output of the section 8.
As shown in FIG. 1, the front section 8 generally comprises two diversity paths 10a and 10b. The path 10a includes an antenna 12a, a down-converter 14a, a local oscillator 16a, a complex multiplier 18a, and a correlator 20a. In exactly the similar manner, the path 10b includes an antenna 12b, a down-converter 14b, a local oscillator 16b, a complex multiplier 18b, and a correlator 20b.
The antennas 12a and 12b receive respectively incoming signals over separate radio transmission paths. The down-converter 14a is supplied with the incoming signal from the antenna 12a and translates the frequency of the receive signal to a predetermined IF (intermediate frequency) signal using the local oscillator 16a. Likewise, the other down-converter 14b is supplied with the incoming signal from the antenna 12b and translates the frequency of the receive signal to the predetermined IF (intermediate frequency) signal using the local oscillator 16b. The output of the down-converter 14a is applied to the multiplier 18a and the correlator 20a, while the output of the other down-converter 14b is applied to the multiplier 18b and the correlator 20b.
The outputs of the multipliers 18a and 18b are combined at a combiner 22. Following this, the output of the combiner 22 is applied to an AGC (automatic gain controller) amplifier 24, the output of which is fed to the above mentioned correlators 20a and 20b and also applied to a DFE (decision feedback equalizer) 26. The equalized output of the DFE 26 is then demodulated at a demodulator (not shown) in a manner well known in the art. The DFE 26 operates such as to equalize waveform distortion caused by multipath fading induced during transmission. For further details of the DFE 26, reference should be made, for example, to U.S. Pat. No. 5,119,401 to Tsujimoto.
Designating the transmitted data (viz., symbol sequences), respectively derived from the down-converters 14a and 14b, as r.sub.n.sup.1 and r.sub.n.sup.2 (n=-00 . . . +00), the following equations are obtained. Throughout the instant disclosure the notation .SIGMA. means summation from n=-00 to n=00. EQU r.sub.n.sup.1 =.SIGMA.h.sub.i.sup.1 a.sub.n-i (1) EQU r.sub.n.sup.2 =.SIGMA.h.sub.i.sup.2 a.sub.n-i (2)
where
h.sub.i.sup.1 : impulse response at time i at the diversity path 10a, PA1 h.sub.i.sup.2 : impulse response at time i at the diversity path 10b, PA1 a.sub.n-i : data transmitted at time (n-i).
The complex multiplier 18a controls the level/phase of the data or signal r.sub.n.sup.1 using a weighting coefficient w.sub.1 applied thereto from the correlator 20a. In a similar manner, the complex multiplier 18b controls the level/phase of the data r.sub.n.sup.2 using a weighting coefficient w.sub.2 applied thereto from the correlator 20b.
As mentioned above, the outputs of the multipliers 18a and 18b are applied to the combiner 22. Designating the data combined at the combiner 22 as y.sub.n, we obtain the following equation (3). ##EQU1## In equation (3), the value of (w.sub.1 h.sub.i.sup.1 +w.sub.2 h.sub.i.sup.2) can be set to unity (viz., 1) because each of the weight coefficients w.sub.1 and w.sub.2 is generated by correlating the output of the AGC amplifier 24, whose level has been normalized at the AGC amplifier 24, with the incoming data. Therefore, the equation (3) can be simplified as follows. EQU y.sub.n =.SIGMA.a.sub.n-i (4)
On the other hand, the weight coefficients w.sub.1 and w.sub.2 are represented by ##EQU2## where * indicates complex conjugate, and the symbol E indicates an evaluation function.
Therefore, the following equation is obtained from equations (3) and (5). EQU y.sub.n =.SIGMA.(h.sub.i.sup.1* .multidot.h.sub.i.sup.1 +h.sub.i.sup.2* .multidot.h.sub.i.sup.2)a.sub.n-1 (6)
In equation (6), the parenthesized portion indicates multiplication of complex conjugate numbers. In other words, equation (6) represents the power-two combination, viz., the maximum ratio combination.
The combiner 22 is well known in the art and hence, a further detailed description thereof will be omitted for brevity.
With the maximum-ratio combining, if the data r.sub.n.sup.2 on the diversity path 10b has a level higher than the data r.sub.n.sup.1 on the other diverty path 10a, the weight coefficient w.sub.2 becomes larger than w.sub.1, and vice versa. That is to say, the combiner 22 operates such as to maximize a signal-to-noise ratio (S/N) of the signal with the higher level. This is the feature of the combining technique.
However, the combiner 22 has suffered from the following difficulty. That is, assuming that the incoming data or signal r.sub.n.sup.2 has been contaminated by an interfering signal such as to exhibit a level higher than the other incoming data r.sub.n.sup.1 (assuming that r.sub.n.sup.1 is a desired signal). In such a case, the combiner 22 operates such as to maximize a signal-to-noise ratio (S/N) in connection with the signal having a higher level (viz., the contaminated signal r.sub.n.sup.2 in this instance). In other words, the maximum-ratio combining is undesirably not carried out on the desired signal r.sub.n.sup.1 but on the interfered signal r.sub.n.sup.2. Summing up, combiner 22 implements diversity combination with respect to the signal with a higher level irrespective of whether this signal is a desired signal or an interfered signal.